Automatic fastening machines are well known and have particular application in the fabricating and assembly of aircraft structures. Such a fastening machine utilizes a numerically controlled transfer head that selectively positions various tools mounted on the machine for sequential operation on a workpiece. In the case of joining two metal workpieces to form a wing panel, the transfer head positions a motorized drill over the workpieces and a hole is drilled therethrough. The transfer head then removes the motorized drill and engages a rivet tool with a large hydraulically-operated cylinder mounted on the fastening machine.
In operation, the rivet tool first picks up a fastener such as a rivet and the hydraulic cylinder actuates to insert the rivet into the hole with both ends of the rivet protruding beyond the workpiece. Heads are formed on the rivet ends to thereby join the workpieces by cooperative interaction between an anvil attached to the hydraulic cylinder and a backup bucking ram, whereupon the transfer head disengages the rivet tool. If desired, the transfer head positions a motorized shave tool over that rivet head formed on the outside surface of the workpiece to mill the upper head of the rivet flush with that surface after rivet upset. In addition, the shave tool can be used to mill the inside surface of the drilled hole to provide a smooth finish prior to rivet insertion.
In some fastening applications, the drilled hole is expanded to a predetermined size before fastener insertion by coldworking the hole. This is done to increase the material strength of the workpiece around the hole to thereby resist localized stress concentrations. Coldworking is the process of plastically deforming metal at a temperature below its annealing point in order to increase the hardness and tensile strength of the metal.
The coldworking assembly consists of a collapsible or split mandrel and a pusher assembly attached to a rod that is axially movable through an internal passage in the mandrel. The mandrel is provided with plurality of longitudinal slots spaced at uniform intervals around the circumference of the mandrel and extending therethrough to provide a plurality of collapsible fingers. The mandrel is hydraulically inserted into the hole and the collapsible fingers expand when the rod is moved through the mandrel passage. An internal locking assembly maintains this expanded mandrel position so that when the mandrel is hydraulically withdrawn, the hole is expanded or coldworked to the predetermined size. An internal release system unlocks the rod for retraction out of the mandrel to collapse the mandrel for a subsequent operation. The coldworking tool is then removed from the vicinity of the hole to enable a motorized reaming and/or countersinking operation to be performed before the fastener is inserted into the hole and upset.
While the split mandrel coldworking tool works well for its intended purpose, there is a problem with wear on the rod and mandrel as these members move with respect to each other during mandrel expansion after insertion into the hole, and again when the rod is removed through the mandrel passage after completing of the coldworking operation. Such wear creates a need for periodic lubrication of the coldworking tool.
In addition, coldworking tool assemblies heretofore available provide a fixed amount of mandrel travel when inserted in the workpiece and therefore the mandrel typically extends a fixed distance beyond the opposite side of the workpiece. Variations in cavity depth of tooling contacting the opposite side of the workpiece can present a problem of the mandrel tip impacting the tooling. Such tooling variation creates a need for controlling the amount of mandrel travel.